This invention relates generally to a method and apparatus for purifying breathing air, and in particular, to an activated carbon monolith structure impregnated with copper, silver, zinc, and molybdenum or the oxides, salts, or complexes thereof, which may be thermally treated, and triethylenediamine.
Filters for purifying breathing air contaminated with toxic or odorous compounds are generally comprised of beds packed with carbon particles. Activated carbon monoliths can be made by forming a structure from a precursor resin, carbonizing, and activating the structure. Such a structure is structurally superior to packed beds made from particulate activated carbon. One disadvantage of typical particulate activated carbon is particle attrition due to particle contact in packed beds, which leads to potential dusting.
Another disadvantage of typical activated carbon packed beds used for filtration is that the pressure drop across these activated carbon beds is too high. Due to the high flow rates used in military applications, such as nuclear, biological, chemical (NBC) filters, a low pressure drop is desirable.
At high flow rates, the increased pressure drop across filters becomes unsatisfactory because the energy required to move the contaminated air stream increases. To mitigate this pressure drop would require a larger bed cross-section or larger particles, both of which reduce the filter efficiency on a weight basis. Furthermore, it is important to realize that because some applications are already limited by the mass transfer efficiency (diffusion) of the toxic compounds to the carbon surface, rather than by the theoretical sorption capacity of the bed, the geometry and adsorption kinetics are of increased importance.
Filtration performance may be enhanced when performed at high temperature. However, the typical activated carbon is susceptible to heat damage at high temperatures. For example, the carbon may burn if the air to be processed is too hot, or if the heat of adsorption or the heat of reaction is too great. Further, typical activated carbon cannot be processed at high temperatures for example, after impregnation, it may not be desirable to heat activated carbon in order to decompose the impregnated species to increase the effective surface area or create a more active species. Therefore, a heat-resistant carbon-based filter would have increased purification performance with a greater degree of safety of operation.
U.S. Pat. No. 4,820,318 to Chang, et al. describes the removal of organic compounds from gas streams using carbon molecular sieves as an adsorbent material. However, the Chang reference does not describe using metal species on an adsorbent material surface for selective removal of compounds from contaminated air.
U.S. Pat. No. 5,063,196 to Doughty, et al. describes an activated carbon adsorbent that is impregnated by what is known in the art as “incipient wetness” or “dry impregnation.” In the incipient wetness method, the metal salts may be dissolved in just the amount of solution required to fill the pore volume of the activated carbon adsorbent. It is known in the prior art (Park and Regalbuto, J. Colloid and Interface Science 175, 1995, 239) that when the carbon and solution are contacted, the pH of the resulting thick slurry will approach the point of zero charge (“PZC”) of the carbon and the interaction of the metal ions with the carbon surface will be weak. The PZC is the pH at which a substrate surface is neutrally charged. At this pH the majority of the —OH groups are neither protonated, which would give rise to a positively charged surface (—OH2+), or deprotonated, which would give rise to a negatively charged surface (—O−). Thus, one cannot control the agglomeration of metals on the surface, as the metals are randomly deposited when the solvent disappears. This means that a large amount of metal remains behind in solution (and later removed as waste) instead of being adsorbed to the surface.
As can be seen, there is a need for an improved apparatus and method for purifying breathing air with high flow rates and low pressure drops. There is also a need for controlling the agglomeration of metals on a surface so that most of the metal is adsorbed to a surface instead of causing excess waste and expense. Furthermore, there is a need for an apparatus and method for NBC applications.